TOKYO — The operator of Japan’s tsunami-crippled nuclear power plant said Tuesday that about 300 tons (300,000 liters, 80,000 gallons) of highly radioactive water have leaked from one of the hundreds of storage tanks there — its worst leak yet from such a vessel.

Tokyo Electric Power Co. said the contaminated water leaked from a steel storage tank at the wrecked Fukushima Dai-ichi plant. TEPCO hasn’t figured out how or where the water leaked, but suspects it did so through a seam on the tank or a valve connected to a gutter around the tank.